1. Field of the Invention
This invention relates generally to techniques for the storage and recovery of magnetically recorded digital information and more particularly, relates to improvements in the readout of digital information recorded on magnetic tape cassettes, disks or diskettes.
2. Description of the Prior Art
Digital information is conventionally recorded on magnetic surfaces by causing a series of flux transitions or reversals to occur on the magnetic surface. The spacing of these flux transitions encodes the data being recorded. One well known problem, exacerbated by the requirements for higher density recording formats, is the problem of peak shift due to density effects.
As density requirements are increased, the flux transitions are recorded more closely together. When transitions are stored too closely together a problem known as pulse crowding or density effects occurs. This phenomenon causes the peaks of closely spaced, or crowded, transitions bounded by wider spaced transitions to move into that wider space between such wider spaced transitions. This movement or spatial shifting is translated into a time shift by the speed of the relative motion between the medium and the read head. The resultant apparent time shift of the peak, known as peak shifting, makes accurate decoding of the spatially encoded digital data more difficult and imposes a fundamental limit on magnetic data recording density.
The same phenomenon also reduces the output amplitude of closely spaced transitions relative to the amplitude of more widely spaced transitions. Both the peak shifting as well as the amplitude loss of closely spaced or crowded flux transitions can be shown from the linear superposition of individual, isolated transitions and causes an effective low pass filtering of the recorded data.
In particular, conventional digital recording techniques such as 1-7 and 2-7 RLL, MFM, GCR, FM, PE, and NRZ coding techniques, encode the magnetic information by controlling the relative positions of, i.e. the spacing between, the magnetic flux transitions. Such conventional encoding techniques may therefore all be considered phase modulation techniques in that the data are decoded by determining the relative time delays between such transitions. A varying time delay within the band of frequencies of interest for this phase modulation, such as may be caused by peak shifting as discussed above or the use of a filter with non-linear phase characteristics, impedes accurate signal decoding and recovery.
Conventional read channels are therefore typically designed for constant group delay characteristics, that is, designed to have linear phase versus frequency characteristics. Fortunately, the equivalent low pass filtering caused by the density effects also usually has a linear phase versus frequency characteristic.
Additionally, digital data recovery systems commonly intentionally utilize frequency discriminating filters, including low pass filtering, to reduce unwanted noise and interference. In such systems, noise is typically introduced into the signals being recovered from several sources. The power of such statistical noise is therefore proportional to the bandwidth of the channel through which the signals to be processed pass. Frequency discriminating filters are used to limit the power of such noise, and to reject interference from higher frequency sources, while passing the signals in the frequency range of the signal being recovered. Typically, the frequency band of interest in the signal being recovered is allowed to pass through in the filters while the noise power is reduced by a low pass filter.
To avoid phase distortion, such low pass filters are conventionally required to be linear phase filters, such as a Bessel filter. However, simple linear phase low pass filters do not usually have sharp amplitude cut offs. That is, such filters typically have a slow amplitude rolloff with increasing frequency so that the amplitude of higher frequency signals is undesirably reduced. The selection of the "cutoff frequency" of such noise and interference reducing filters is therefore usually a compromise between noise reduction and the ability to pass the higher frequencies in the signals being recovered. The loss of such high frequencies, caused by the above noted, unwanted slow rolloff of the linear phase low pass filters used to reduce noise power in the band of interest, results in further peak shifting similar in effect to those caused by density effects, always in a direction which aggravates this problem.
The analog readback signal from the read head is a complex waveform containing the harmonics and sidebands necessary to support the frequency shifts caused by the intentionally uneven peak spacing used to encode the data. The loss of even some of these higher frequency signals, by for example the equivalent low pass filtering resulting from density effects, distorts the relative time between successive peaks in the readback signal and thereby impedes accurate data recovery.
One way of measuring or comparing the relative frequency responses of such systems is the figure of merit known as resolution. Resolution is often defined as the ratio of the amplitude of the analog readback signal at the highest transition rate or density in the coding scheme being used to the amplitude of the readback signal at the lowest transition density in the coding scheme being used. That is, resolution is defined as the amplitude ratio of the highest fundamental frequency to the lowest fundamental frequency for the code being used. Resolution is therefore a two point measurement of the frequency response of the noise filter, media and head as a system.
One improvement in digital magnetic data recovery has been the use of slimming filters: a class of linear phase filters which have a rising amplitude response that can be controlled to mirror and therefore compensate for the loss of some of the high frequency portions of the recovered signals due to density and other effects.
Slimming filters are implemented as additive filters in that a circuit providing a linear phase transfer function having a rising amplitude as a function of frequency is added to a low pass filter with a matching phase characteristic to mirror the undesired high frequency rolloff of the media and head system due to density effects. Slimming filters can be implemented in various known ways; such as by subtracting signals from the input and output taps of a terminated delay line from the center tap of that line; by subtracting, from the output of a delay line whose source is terminated and whose output is at a high impedance, the signal at the tap between the source termination and the input of the line; by subtracting the output of a second order high pass filter from the output of a low pass filter with matching phase characteristic; by cascading a system consisting of a second order bandpass filter subtracted from a matching low pass filter; or by subtracting the output of a fourth order band-pass filter from a fourth order low pass filter with matching phase characteristic. Various other known techniques, including the addition of appropriately signed even derivatives, are also available for use in slimming filters.
When high pass or band-pass filters are used, the Q and natural resonant frequencies of such filters are selected to make these elements, in combination with the other elements present in such filters, operate to form a linear phase filter. When subtraction is used, the shape of the amplitude response can be tailored by adjusting how much of one channel is subtracted from the other. In fact, in accordance with U.S. Pat. No. 4,306,257 issued in 1981 to the inventor hereof, the subtracting may be made adaptive. In particular, as used in that patent, the filter response was tailored to the varying bit density of a disk.
What are needed are further enhancements of the techniques available for use with higher data densities to eliminate or reduce density effects.